The present invention relates to solid-state display devices and means to create a broader display gamut than is conventionally achieved or to optimize the gamut for a particular application.
Digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes and solid state light emitters such as LEDs. In most cases, each display element or pixel, is composed of red, green, and blue colored subpixels. By combining the illumination of each of these three subpixels in an additive color system, a wide variety of colors can be achieved. These are often represented as a triangular space on a CIE chromaticity diagram whose three defining points represent visual tristimulus values. In FIG. 2, a generic CIE chromaticity diagram 10 shown with a triangular area 12 representing the colors defined within the NTSC specification defined by three color points 14 at the corners of the triangle 12. This triangular area is called the color gamut of the display device. Other coordinate triplets can represent other color gamut specifications, for example the colors to which retinal pigments are responsive. In the printing industry, the use of multiple layers using combinations of inks in the subtractive primary colors (cyan, magenta and yellow) as well as black (K), is well known as are conversions from the typical 3-color additive systems (R,G,B) used for displays to the four-color subtractive printing processes, for example, cyan, yellow, magenta, and black (CYMK); (see European Patent EP 0 586 139 B 1, by Litvak et al., May 27, 1998, entitled Printing Apparatus and Method for More than Five Colours). It is also known to employ even more colored inks, such as international orange and fluorescent inks in the printing process.
Despite the versatility of known digital image display devices, these devices cannot display every color that can be seen by a human, that is, the color gamut of these devices is limited. For example, conventional cathode ray tube (CRT) displays are limited to the colors expressed by combinations of the phosphors coated on their vacuum tube. That is, they cannot reproduce colors outside the gamut defined by their chromaticity coordinates. Other types of displays (e.g. liquid crystal devices LCDs) have similar limitations on their light sources and/or filter elements. Typically, the colors achieved by conventional digital image display devices produce colors having limited saturation. Generally, the available colors are matched as best they can by applying knowledge of the input scene or capture device and the display device (see U.S. Pat. No. 5,243,414, issued Sep. 7, 1993 to Dalrymple et al., entitled Color Processing System, and U.S. Pat. No. 5,844,699, issued Dec. 1, 1998 to Usami et al., entitled Color Image Processing Apparatus). Often, this is achieved through the use of standard color spaces with lookup tables that convert images from one color space to another as needed (see for example U.S. Pat. No. 5,614,925 issued Mar. 25, 1997 to Braudaway et al., entitled Method and Apparatus for Creating and Displaying Faithful Color Images on a Computer Display, and the International Color Consortium specifications at http://www.color.org).
Today, most images are displayed on a CRT or a liquid crystal display and are described as combinations of red, green, and blue with a numerical value (usually from 0 to 255 equivalent to 8 bits) for each pixel color. (For example, see EP 0 513 173 B1, by Shapiro et al., Mar. 3, 1997, entitled Liquid Crystal Display Panel System and Method of Using Same). There are thus 256xc3x97256xc3x97256 or about 16 million possible color combinations. Although this appears to be a lot of color combinations, the combinations cannot express some colors, even if additional bits are added because the additional bits provide only a finer control of the amount of each of the three colors used in the display, not the gamut of colors that can be produced by the display. Therefore the color gamut is limited and a desired color often cannot be properly displayed by a device due to the inherent limitations in the color of the subpixels (e.g. phosphors). The colors of the subpixels can vary and various implementations use different combinations thus modifying the color gamut of the device (see U.S. Pat. No. 5,184,114, issued Feb. 2, 1993 to B. Brown, entitled Solid State Color Display System and Light Emitting Diode Pixels Therefor).
Mechanisms for ameliorating the gamut limitations of a CRT by equalizing the display drive signals, for example, are known (see U.S. Pat. No. 4,994,901, issued Feb. 19, 1991 to Parulski et al, entitled Method and Apparatus for Increasing the Gamut of an Additive Display Driven from a Digital Source). Alternatively, ways to optimize the use of the available colors are known (see U.S. Pat. No. 5,614,925). However, these techniques do not address the fundamental issue of limited color gamut in the display device. There is a need therefore for an improved digital image display device having an extended color gamut.
The need is met by providing a digital color image display device, that includes a plurality of pixels, each pixel having a plurality of subpixels with at least one of the subpixels producing a color other than red, green or blue.
According to the invention, each pixel can be composed of more than red, green, and blue emitters or colors other than red, green, and blue. The emitters are controlled using conventional means. Where a digital color signal is not matched to the available pixel colors, a digital color transformation device calculates a suitable combination of available colors to match the desired color representation.
Advantages
The advantages of this invention are a digital image display device with a wider and different range of colors providing a wider color gamut to the user. The wider gamut provides superior image rendition and superior calibration capabilities. Alternatively, an optimized sub-gamut can be used to increase efficiency and lifetime of the device by making one or more of the colors within the gamut defined by the other colors. The device can accept a variety of color specifications. By transforming conventional color specification signals, the device can accept images from existing systems while providing superior color rendition and fidelity.